On determining soot maturity: A review of the role of microscopy- and spectroscopy-based techniques

Trade-off between soot and NO emissions during enclosed spray combustion of jet fuel

Aviation emissions of soot and nitrogen oxides are strictly regulated as they adversely impact human health and the environment. Jet fuel combustion conditions that decrease one pollutant concentration increase the other. Although it is not impossible to achieve both low soot and NOx through clever design, it is hard to simultaneously reduce both. Although it is difficult to study such conditions due to high temperatures and gas flowrates of aircraft engines, recently it was shown that Enclosed Spray Combustion (ESC) of jet fuel results in soot with similar characteristics to that from aircrafts making ESC an attractive unit for studying aviation-like emissions. Furthermore, judicious swirl-injection of air downstream of the ESC burner drastically reduces soot emissions. Here the trade-off between NO and soot emissions during combustion of jet fuel is studied for the first time, to the best of our knowledge, accounting for the detailed structure of soot. Injecting air shortly after the ESC burner decreases soot but increases NO emissions, while such injection further downstream has the inverse outcome. This interplay between soot and NO emissions was correlated quantitatively with the gas temperature shortly after air injection. Consequently, combustion conditions for an optimal trade-off between soot and NO emissions for the ESC conditions studied here are identified that are at or below the lowest NOx emissions per unit mass of fuel from existing aircraft engines.

Cold Plasma for the Modification of the Surface Roughness of Microparticles

Cold plasma treatment is commonly used for sterilization. However, another potential of cold plasma treatment is surface modification. To date, several efforts have been directed toward investigating the effect of cold plasma treatment in modifying the surfaces of films. Here, the impact of suspension properties and parameters of cold plasma treatment on the changes of surfaces of monodisperse polymeric microparticles is tested. The plasma treatment did not touch the surface chemistry of the monodisperse polymeric microparticles. The concentration of suspensions of 1 mg/mL was determined to relate to a stronger effect of the plasma treatment on the roughness of the microparticles. Microparticles with an average diameter of 20 μm show a roughness increase with the plasma treatment time. However, a plasma treatment time longer than 15 min damages the microparticles, as observed in particles with an average diameter of 20 and 50 μm. We finally prototyped monodisperse microparticles to deliver drugs to the nasal mucosa by studying the effect of roughness in their (undesired) self-adhesion and (desired) adhesion with tissue. A moderate roughness, with an average peak-to-valley distance of 500 nm, appears to be the most effective in reducing the detachment forces with nasal tissue by up to 5 mN.

Raman spectroscopy for profiling physical and chemical properties of atmospheric aerosol particles: A review

Atmosphere aerosols have significant impact on human health and the environment. Aerosol particles have a number of characteristics that influence their health and environmental effects, including their size, shape, and chemical composition. A great deal of difficulty is associated with quantifying and identifying atmospheric aerosols because these parameters are highly variable on a spatial and temporal scale. An important component of understanding aerosol fate is Raman Spectroscopy (RS), which is capable of resolving chemical compositions of individual particles. This review presented strategic techniques, especially RS methods for characterizing atmospheric aerosols. The nature and properties of atmospheric aerosols and their influence on environment and human health were briefly described. Analytical methodologies that offer insight into the chemistry and multidimensional properties of aerosols were discussed. In addition, perspectives for practical applications of atmospheric aerosols using RS are featured.

Advances in soot particles from gasoline direct injection engines: A focus on physical and chemical characterisation

With an increasing market share of gasoline direct injection (GDI) vehicles, high particulate emissions of GDI engines are of increasing concern due to their adverse impacts on both human health and the ecological environment. A thorough understanding of GDI nanoparticulate properties is required to develop advanced particulate filters and assess the exhaust toxicity and environmental impacts. To this end, this paper aims to provide a comprehensive review of the physical and chemical characteristics of GDI nanoparticles from a distinctive perspective, including soot oxidation reactivity, morphology, nanostructure, surface chemistry, chemical components, and their correlations. This review begins with a brief description of nanoparticle characterisation methods. Then, the nanoparticle characteristics of GDI engines are reviewed with the following aspects: in-cylinder soot, exhaust particulate features, and a comparison between GDI and diesel nanoparticles. Previous studies showed that exhaust nanoparticle presents a more stable nanostructure and is less prone to oxidation if compared with in-cylinder soot. Additionally, GDI particles are less-ordered, more inorganic and metallic containing, and more reactive than diesel particles. Afterwards, the impacts of engine operating parameters and aftertreatments on GDI soot features are discussed in detail. Finally, the conclusions and future research recommendations are presented.

Automated particle recognition for engine soot nanoparticles

A pre-trained convolution neural network based on residual error functions (ResNet) was applied to the classification of soot and non-soot carbon nanoparticles in TEM images. Two depths of ResNet, one 18 layers deep and the other 50 layers deep, were trained using training-validation sets of increasing size (containing 100, 400 and 1400 images) and were assessed using an independent test set of 200 images. Network training was optimised in terms of mini-batch size, learning rate and training length. In all tests, ResNet18 and ResNet50 had statistically similar performances, though ResNet18 required only 25-35% of the training time of ResNet50. Training using the 100-, 400- and 1400-image training-validation sets led to classification accuracies of 84%, 88% and 95%, respectively. ResNet18 and ResNet50 were also compared for their ability to categorise soot and non-soot nanoparticles via a fivefold cross-validation experiment using the entire set of 800 images of soot and 800 images of non-soot. Cross-validation was repeated 3 times with different training durations. For all cross-validation experiments, classification accuracy exceeded 91%, with no statistical differences between any of the network trainings. The most efficient network was ResNet18 trained for 5 epochs, which reached 91.2% classification after only 84 s of training on 1600 images. Use of ResNet for classification of 1000 images, the amount suggested for reliable characterisation of soot sample, requires <4 s, compared with >30 min for a skilled operator classifying images manually. Use of convolution neural networks for classification of soot and non-soot nanoparticles in TEM images is highly promising, particularly when manually classified data sets have already been established.

Changes of diesel particle diameter and surface area distributions by non-thermal plasma

A wide literature has demonstrated that internal combustion engines are the main responsible for the emission of fine particles in urban areas. Within this scope, ultrafine particles within diesel exhausted gas have been widely proven to exert a significantly harmful impact on human health and environment. This scenario has led the research community to turn the attention from particle mass to diameter and surface area. In this paper, non-thermal plasma (NTP) technology was applied to a heavy duty diesel engine. Chemical reactions of diesel particles in plasma zone were analyzed. Additionally, variation in diesel particles' number and surface area distributions, engendered by above reactions, were thoroughly investigated. The results showed that diesel exhausted particles experienced oxidation, aggregation, and crush because of enhanced plasma transports and active species in plasma zone. NTP presents excellent reduction effectiveness of diesel particles covering different sizes. Being more than 50%, the most considerable surface area concentration drop was found in correspondence of 1800 RPM. Differently, the lowest drop of surface area concentration was seen at 1200 RPM. As a result of the NTP actions, surface area concentration distributions were almost the same for diameters being larger than 0.5 μm at different engine modes, except at 900 RPM. This research made a foundation of dropping particle emissions and evaluating the effectiveness of NTP dropping particle harms to human health.

Temperature Sensing with Thin Films of Flame-Formed Carbon Nanoparticles

A porous nanostructured film of flame-formed carbon nanoparticles has been produced with a one-step procedure. The morphological and structural characteristics of the film have been characterized by atomic force microscopy and Raman spectroscopy. The electrical resistance as a function of the temperature has been investigated in the range from ambient temperature to 120 °C. A nonmetallic behavior has been observed, with a monotonic decrease of the film resistance as temperature increases. Electrical conduction is explained in terms of charge carriers tunneling and percolation between the carbon grains and is not perfectly described by an Arrhenius behavior. A negative temperature coefficient of resistance (TCR) of the order of −100 × 10−4 K−1 has been measured. The high absolute TCR value, together with the ease of material microfabrication processing and biocompatibility of the carbon material make this film ideal for temperature sensing in many environments. A functional relationship between resistance and temperature, which is necessary for practical applications, has been finally derived. A very good agreement between experimental data and fit is obtained with a fifth order polynomial.

Can Plasma Surface Treatment Replace Traditional Wood Modification Methods?

Wood modification is an excellent and increasingly used method to expand the application of woody materials. Traditional methods, such as chemical or thermal, have been developed for the targeted improvement of some selected properties, unfortunately typically at the expense of others. These methods generally alter the composition of wood, and thus its mechanical properties, and enhance dimensional stability, water resistance, or decrease its susceptibility to microorganisms. Although conventional methods achieve the desired properties, they require a lot of energy and chemicals, therefore research is increasingly moving towards more environmentally friendly processes. The advantage of modern methods is that in most cases, they only modify the surface and do not affect the structure and mechanical properties of the wood, while reducing the amount of chemicals used. Cold plasma surface treatment is one of the cheapest and easiest technologies with a limited burden on the environment. In this review, we focus on cold plasma treatment, the interaction between plasma and wood compounds, the advantages of plasma treatment compared to traditional methods, and perspectives.

Laser-induced incandescence for non-soot nanoparticles: recent trends and current challenges

Laser-induced incandescence (LII) is a widely used combustion diagnostic for in situ measurements of soot primary particle sizes and volume fractions in flames, exhaust gases, and the atmosphere. Increasingly, however, it is applied to characterize engineered nanomaterials, driven by the increasing industrial relevance of these materials and the fundamental scientific insights that may be obtained from these measurements. This review describes the state of the art as well as open research challenges and new opportunities that arise from LII measurements on non-soot nanoparticles. An overview of the basic LII model, along with statistical techniques for inferring quantities-of-interest and associated uncertainties is provided, with a review of the application of LII to various classes of materials, including elemental particles, oxide and nitride materials, and non-soot carbonaceous materials, and core-shell particles. The paper concludes with a discussion of combined and complementary diagnostics, and an outlook of future research.

Laser-induced incandescence for non-soot nanoparticles: recent trends and current challenges

Laser-induced incandescence (LII) is a widely used combustion diagnostic for in situ measurements of soot primary particle sizes and volume fractions in flames, exhaust gases, and the atmosphere. Increasingly, however, it is applied to characterize engineered nanomaterials, driven by the increasing industrial relevance of these materials and the fundamental scientific insights that may be obtained from these measurements. This review describes the state of the art as well as open research challenges and new opportunities that arise from LII measurements on non-soot nanoparticles. An overview of the basic LII model, along with statistical techniques for inferring quantities-of-interest and associated uncertainties is provided, with a review of the application of LII to various classes of materials, including elemental particles, oxide and nitride materials, and non-soot carbonaceous materials, and core-shell particles. The paper concludes with a discussion of combined and complementary diagnostics, and an outlook of future research.

Sonochemical Preparation of a Magnet-Responsive Fe3O4@ZIF-8 Adsorbent for Efficient Cu2+ Removal

This work presents a novel approach to synthesizing magnetic core-shell nanocomposites, consisting of magnetic nanoparticles and a metal-organic framework, for environmental applications. The synthesis is based on the encapsulation of magnetic Fe₃O₄ nanoparticles with microporous zeolitic imidazolate framework-8 (ZIF-8) nanocrystals via ultrasonic activation under a continuous supply of precursor solutions. This sonochemical approach is proven to be a fast, cost-effective, and controllable route for the preparation of magnet-responsive Fe₃O₄@ZIF-8 nanoparticles with a core-shell structure. The functional nanomaterial possesses a high content of ZIF-8 and combined micro/mesoporosity, and thus can be used as adsorbents that can be easily separated using a magnet. In particular, the sonochemically prepared Fe₃O₄@ZIF-8 exhibits significant adsorption performance for the removal of copper ions from water: a short adsorption time (10 min), high maximum uptake capacity (345 mg g⁻¹), and excellent removal efficiency (95.3%). These performances are interpreted and discussed based on the materials characteristics of Fe₃O₄@ZIF-8 established by microscopy, gas sorption, X-ray diffraction, and thermal analysis.

Laser-induced incandescence for non-soot nanoparticles: recent trends and current challenges

Laser-induced incandescence (LII) is a widely used combustion diagnostic for in situ measurements of soot primary particle sizes and volume fractions in flames, exhaust gases, and the atmosphere. Increasingly, however, it is applied to characterize engineered nanomaterials, driven by the increasing industrial relevance of these materials and the fundamental scientific insights that may be obtained from these measurements. This review describes the state of the art as well as open research challenges and new opportunities that arise from LII measurements on non-soot nanoparticles. An overview of the basic LII model, along with statistical techniques for inferring quantities-of-interest and associated uncertainties is provided, with a review of the application of LII to various classes of materials, including elemental particles, oxide and nitride materials, and non-soot carbonaceous materials, and core-shell particles. The paper concludes with a discussion of combined and complementary diagnostics, and an outlook of future research.

Transport and Separation of the Silver Ion with n-decanol Liquid Membranes Based on 10-undecylenic Acid, 10-undecen-1-ol and Magnetic Nanoparticles

This paper presents a transport and recovery of silver ions through bulk liquid membranes based on n-decanol using as carriers 10-undecylenic acid and 10-undecylenyl alcohol. The transport of silver ions across membranes has been studied in the presence of two types of magnetic oxide nanoparticles obtained by the electrochemical method with iron electrodes in the electrolyte with and without silver ions, which act as promoters of turbulence in the membrane. Separation of silver ions by bulk liquid membranes using 10-undecylenic acid and 10-undecylenyl alcohol as carriers were performed by comparison with lead ions. The configuration of the separation module has been specially designed for the chosen separation process. Convective-generating magnetic nanoparticles were characterized in terms of the morphological and structural points of view: scanning electron microscopy (SEM), high-resolution SEM (HR-SEM), energy dispersive spectroscopy analysis (EDAX), Fourier Transform InfraRed (FTIR) spectroscopy, thermal gravimetric analysis (TGA), differential scanning calorimetry and magnetization. The process performance (flux and selectivity) was tested were tested for silver ion transport and separation through n-decanol liquid membranes with selected carriers. Under the conditions of the optimized experimental results (pH = 7 of the source phase, pH = 1 of the receiving phase, flow rate of 30 mL/min for the source phase and 9 mL/min for the receiving phase, 150 rot/min agitation of magnetic nanoparticles) separation efficiencies of silver ions of over 90% were obtained for the transport of undecenoic acid and about 80% for undecylenyl alcohol.

Influence of Onion-like Carbonaceous Particles on the Aggregation Process of Hydrocarbons

Molecular dynamics simulations are performed to characterize the nucleation behavior of organic compounds in the gas phase. Six basic molecular species are considered-ethylene, propylene, toluene, styrene, ethylbenzene, and para-xylene-in interaction with onion-like carbon nanostructures that model soot nanoparticles (NPs) at room temperature. We identify a shell-to-island aggregation process during the physisorption of aromatic molecules on the soot surface: The molecules tend to first cover the NP in a shell, on top of which additional adsorbates form island-shaped aggregates. We present results for the binding energy, suggesting that the NPs lead to the formation of more stable molecular aggregates in comparison with the pure gas phase. Our findings describe a plausible microscopic mechanism for the active role of soot in the formation and growth of organic particulate matter.

Resistive Switching Phenomenon Observed in Self-Assembled Films of Flame-Formed Carbon-TiO2 Nanoparticles

Nanostructured films of carbon and TiO₂ nanoparticles have been produced by means of a simple two-step procedure based on flame synthesis and thermophoretic deposition. At first, a granular carbon film is produced on silicon substrates by the self-assembling of thermophoretically sampled carbon nanoparticles (CNPs) with diameters of the order of 15 nm. Then, the composite film is obtained by the subsequent thermophoretic deposition of smaller TiO₂ nanoparticles (diameters of the order of 2.5 nm), which deposit on the surface and intercalate between the carbon grains by diffusion within the pores. A bipolar resistive switching behavior is observed in the composite film of CNP-TiO₂. A pinched hysteresis loop is measured with SET and RESET between low resistance and high resistance states occurring for the electric field of 1.35 × 10⁴ V/cm and 1.5 × 10⁴ V/cm, respectively. CNP-TiO₂ film produced by flame synthesis is initially in the low resistive state and it does not require an electroforming step. The resistance switching phenomenon is attributed to the formation/rupture of conductive filaments through space charge mechanism in the TiO₂ nanoparticles, which facilitate/hinder the electrical conduction between carbon grains. Our findings demonstrate that films made of flame-formed CNP-TiO₂ nanoparticles are promising candidates for resistive switching components.

Synthesis of Fe0/Fe3O4@porous carbon through a facile heat treatment of iron-containing candle soots for peroxymonosulfate activation and efficient degradation of sulfamethoxazole

Developing highly efficient, reusable, non-toxic and low-cost catalysts is of great importance for persulfate-based advanced oxidation processes (AOPs). In this work, ferrocene was mixed into paraffin to prepare a candle, and the iron-containing candle soots were collected and heated at 500 °C~900 °C under N₂ atmosphere for 1 h to prepare magnetically recyclable Fe⁰/Fe₃O₄@porous carbon (Fe⁰/Fe₃O₄@PC) catalysts. The Fe⁰/Fe₃O₄@PC-700 obtained after pyrolysis at 700 °C exhibited the best catalytic activity for sulfamethoxazole (SMX) degradation. 10 mg/L SMX could be completely degraded within 10 min by 0.2 g/L of Fe⁰/Fe₃O₄@PC-700 and 0.5 mM PMS at pH 5.0. The carbon shell effectively inhibited the Fe leaching of Fe⁰/Fe₃O₄@PC-700, and 99.73% of Fe was retained after five consecutive cycles. In the Fe⁰/Fe₃O₄@PC-700/PMS system, SMX was degraded through the sulfate radical (SO₄·^¯), hydroxyl radical (·OH), superoxide radical (O₂·^¯) dominated radical pathway, and the singlet oxygen (¹O₂) dominated non-radical pathway. The coexisting inorganic ions and natural organic matters (NOM) in actual water inhibited the degradation of SMX. Finally, four possible degradation pathways were proposed based on the degradation intermediates of SMX. This work provides a facile heat treatment of iron-containing candle soots strategy to prepare the metal@carbon catalysts for PMS-based AOP.

Nanostructure and reactivity of soot particles from open burning of household solid waste

The main purpose of this work was to quantify and characterize chemically and morphologically the emission of soot particles from the open burning of several common solid waste including paperboard, wood, peel, chemical fiber, polyethylene (PE) and polyvinyl chloride (PVC). The experiment was conducted in a laboratory-scale open-burning combustor with a dilution sampling system to obtain soot particles. The thermogravimetric profiles (TGA) showed an increasing order of oxidation reactivity: PE > PVC > fiber > paper ≈ peel > wood. High resolution transmission electron microscopy (HRTEM) images revealed more detailed information about the morphology and the particle size of soot aggregates. Subsequent quantification of nanostructure by fringe analysis showed that plastics generated soot particles with the looser carbon layers with higher tortuosity compared to the three kind of biomass. Raman spectroscopy further confirms the observed differences. In addition, wood soot exhibited the highest content of C-OH group (17.5%) among the six samples (X-Ray photoelectron spectroscopy, XPS), whereas PE and PVC soot exhibited the highest absorption peaks of aliphatic C-H groups (Fourier transform infrared spectroscopy, FTIR). Comparative analysis revealed that the interlayer distance was more important on the evaluation of reactivity than soot morphologies. The present work concluded that the physiochemical characteristics of soot particles releasing during open burning are strongly depending on waste composition and provided new data for the understanding of soot emissions from open burning.

Biochar Nanocomposite Derived from Watermelon Peels for Electrocatalytic Hydrogen Production

Water splitting is the most potential method to produce hydrogen energy, however, the conventional electrocatalysts encounter the hindrances of high overpotential and low hydrogen production efficiency. Herein, we report a carbon-based nanocomposite (denoted as CCW-x, x stands for the calcination temperature) derived from watermelon peels and CoCl₂, and the as-synthesized CCW-x is used as the electrocatalyst. The overpotential and the Tafel slope of CCW-700 for oxygen evolution reaction (OER) is 237 mV at 10 mA cm^-2 and 69.8 mV dec^-1, respectively, both of which are lower than those of commercial RuO₂. For hydrogen evolution reaction (HER), the overpotential of CCW-700 (111 mV) is higher than that of the widely studied Pt/C (73 mV) but still lower than those of lots of carbon-based nanomaterials (122-177 mV). In the light of CCW-700 is highly active for both OER and HER, we assembled a water-splitting electrocatalyst by employing nickel foam loaded with CCW-700 as the anode and cathode in 1 M KOH. The water-splitting voltage is only 1.54 V for the CCW-700//CCW-700 electrodes and 1.62 V for the RuO₂//Pt/C ones. Therefore, the so-denoted CCW-x powder possesses good electrocatalytic hydrogen production efficiency.

In-flow optical characterization of flame-generated carbon nanoparticles sampled from a premixed flame

In this work, the optical absorption properties of carbon nanoparticles are investigated by applying in-flow extinction and laser-induced incandescence measurements. Carbon nanoparticles are produced in an ethylene/air premixed flame and sampled at different heights above the burner. From extinction measurements, the absorption coefficient is obtained in a wide spectral range, considering the negligible scattering under our experimental conditions. With the application of Tauc plot the optical band gap is evaluated at the sampling heights under analysis. The increase of this value with the decrease in the height is consistent with the quantum confinement effect detected in the inception region of the flame. Two-color laser induced incandescence measurements are performed at relatively high laser fluence. The fluence curves, given by the particle temperature under laser irradiation versus laser fluence, are also obtained. A significant difference in the optical properties of these particles is observed by changing the sampling height. Moreover, considering the fluence curve in the low laser fluence regime, the refractive index absorption function E(m) is evaluated at an excitation wavelength of 1064 nm. Finally, the knowledge of the behavior of the absorption coefficient in a wide spectral range allows retrieving the values and the behavior of E(m) as a function of wavelength.

Thermally treated candle soot as a novel catalyst for hydrogen peroxide in-situ production enhancement in the bio-electro-Fenton system

Thermally treated candle soot (TCS) was used as a two-electron (2e¯) oxygen reduction reaction (ORR) catalyst to in situ produce H₂O₂ in a bio-electro-Fenton (BEF) system. Compared with the pristine candle soot (CS), TCS showed larger Brunauer-Emmett-Teller (BET) surface area (102.54 m² g^-1 vs. 61.79 m² g^-1), higher mesoporous ratio (50.39% vs. 34.98%), and improved hydrophilicity. X-ray photoelectron spectra (XPS) results revealed that the C-O-C was the dominant oxygen-containing group of the CS, and its percentage reached at 80.55%. However, the C-O-C ratio of the TCS decreased to 48.93%, whilst it's CO and OC-O ratios significantly increased to 27.92% and 23.15%. The TCS showed a high H₂O₂ selectivity (87.5%∼97.0%) at the neutral pH condition, which was much higher than that of the commonly used carbon black (CB) catalyst. Finally, the H₂O₂ concentration maxima (C_max-_H2O2) of the bio-electro-Fenton system running with the TCS air-cathode (BEF-TCS) achieved at 32.02 mg/L, which was 6.29 times higher than that of the BEF-CB (5.09 mg/L). The removal and mineralization ratios of the SMX in the BEF-TCS reached at 83.0% and 79.0%, respectively. This paper reported a novel 2e¯ ORR electro-catalyst which was low-cost, easily available and highly efficiency.

Spray-On Nanocomposite Coatings: Wettability and Conductivity

Nanocomposite coatings, i.e., a combination of nanocompounds, and a polymer matrix together with suitable additives and solvents is a very versatile method for producing multifunctional coatings. Some of the most desired coating properties have a high repellency to liquids (e.g., superhydrophobic and/or superoleophobic) and electrical and thermal conductivities. From a practical perspective, coatings that can be sprayed are very suitable for large-scale production, conformity, and reduced time and cost. Carbon-based, metallic, and ceramic are the three groups of nanocompounds commonly used to formulate spray-on nanocomposite coatings. In this invited feature article, we discuss the applications, advantages, and challenges of using such nanocompounds to produce coatings with good water repellency or/and elevated electrical or/and thermal conductivities. We also discuss the role of additives and solvents briefly in relation to the properties of the coatings. Important spraying parameters, such as stand-off distance and its influence on the final coating properties, will also be examined. Our overall aim is to provide a guideline for the production of practical multifunctional nanocomposites utilizing carbon-based, metallic, or ceramic nanoparticles or nanofibers that covers both aspects of in-air wettability and conductivity under one umbrella.